PCI: Check for child busses which use more bus numbers than allocated

pci_scan_child_bus can (potentially) return a bus number higher than the
subordinate value of the child bus. Possible reasons are that bus numbers
are reserved for SR-IOV or for CardBus (SR-IOV is done without checks and
the CardBus checks are sketchy at best).

We clamp the returned value to the actual subordinate value and print a
warning if too many bus numbers are reserved.

[bhelgaas: whitespace]
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>